Validate fairmint quantity before comparisons

[caydyan]

Supersedes #3388 with the same fix rebased onto the current develop branch.

fairmint.validate() recorded quantity must be an integer for non-integer quantities, but then continued into fairminter-specific arithmetic and comparisons when the fairminter existed. For a paid fairminter, a value like "10" could reach quantity <= 0 and raise a Python TypeError instead of returning the compose-level validation error.

This PR stops fairmint validation before any quantity arithmetic when the quantity is not an integer.

Changes:

• Tracks invalid non-integer fairmint quantity before looking up the fairminter.
• Preserves the existing fairminter not found behavior when the asset is missing.
• Returns the recorded quantity validation problem before arithmetic/comparison logic for existing fairminters.
• Adds regression coverage for validate and compose paths.

Tests:

• python -m pytest counterpartycore/test/units/messages/fairmint_test.py::test_validate counterpartycore/test/units/messages/fairmint_test.py::test_compose -q
• python -m pytest counterpartycore/test/units/messages/fairmint_test.py -q
• python -m ruff check counterpartycore/lib/messages/fairmint.py counterpartycore/test/units/messages/fairmint_test.py
• python -m ruff format --check counterpartycore/lib/messages/fairmint.py counterpartycore/test/units/messages/fairmint_test.py
• git diff --check

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